It is known to place a sensor mat in the passenger seat of an automobile in order to obtain size and weight information relating to the occupant of the passenger seat. The sensor mat has sensors constructed as force-dependent resistors arranged to form a matrix having rows and columns. Three-dimensional information can be obtained from the mat by considering the X and Y positions of the activated force-dependent resistors, and by considering the resistance values of these activated force-dependent resistors. The weight and size of the occupant can accordingly be estimated, and this information is useful in determining whether to activate an airbag, and in determining what force should be used if activating the airbag. For example, if the pressure profile indicates that a small child is sitting in the passenger seat, and an event occurs which indicates that the passenger airbag should be triggered, then the passenger airbag can be triggered with less force than would be the case if an average size adult were sitting in the passenger seat. Alternatively, the airbag may not be triggered at all.
Typically, a given force-dependent resistor of a sensor mat will have a certain resistance value in a non-activated state, and when activated, the resistance value will decrease proportionally to the applied force. It is difficult to manufacture a sensor mat such that the non-activated resistance values of the force-dependent resistors have values that lie within a narrow tolerance band. It is also known that the resistance values vary with time and with temperature, and it is desirable to be able to take these variances into account when evaluating signals obtained from the sensor mat. In addition, various leakage currents can occur which also affect the ability to accurately evaluate the signals obtained from the sensor mat. In order to be able to accurately evaluate signals that are obtained from the sensor mat, the actual resistance values of the force-dependent resistors in the sensor mat must be accurately determined.
It is accordingly an object of the invention to provide a sensor mat configuration which overcomes the above-mentioned disadvantages of the prior art apparatus of this general type.
In particular, it is an object of the invention, to provide a sensor mat configuration that enables one to accurately determine the resistance values of the force-dependent resistors such that the known variations from the desired resistance values can be compensated for and the pressure profile of the force-dependent resistors can be accurately obtained.
With the foregoing and other objects in view there is provided, in accordance with the invention, a sensor mat configuration that includes a sensor mat having row lines, column lines, and a plurality of force-dependent resistors connected between the row lines and the column lines to form a matrix having rows and columns. The sensor mat configuration also includes a printed circuit board having a first plurality of fixed resistors. Each one of the first plurality of the fixed resistors has a first terminal and a second terminal. The first terminal of each one of the first plurality of the fixed resistors is connected to a respective one of the column lines and the second terminals of the first plurality of the fixed resistors are connected together, thereby forming an additional row of the matrix.
In accordance with an added feature of the invention, the printed circuit board and the first plurality of the fixed resistors are not integral parts of the sensor mat.
In accordance with an additional feature of the invention, the printed circuit board includes a second plurality of fixed resistors; each one of the second plurality of the fixed resistors has a first terminal and a second terminal. The first terminal of each one of the second plurality of the fixed resistors is connected to a respective one of the row lines and the second terminals of the second plurality of the fixed resistors are connected together, thereby forming an additional column of the matrix.
In accordance with another feature of the invention, the sensor mat includes a plurality of fixed column resistors for checking whether interruptions exist on the column lines and a plurality of fixed row resistors for checking whether interruptions exist on the row lines. Each one of the plurality of the fixed column resistors of the sensor mat has a first terminal connected to a respective one of the column lines. Each one of the first plurality of the fixed column resistors of the sensor mat has a second terminal. The second terminal of each one of the first plurality of the fixed column resistors of the sensor mat are connected together. Each one of the plurality of the fixed row resistors of the sensor mat has a first terminal connected to a respective one of the row lines. Each one of the first plurality of the fixed row resistors of the sensor mat has a second terminal. The second terminals of the first plurality of the fixed row resistors of the sensor mat are connected together.
In accordance with a further feature of the invention, the printed circuit board includes a second plurality of fixed resistors; and each one of the second plurality of the fixed resistors has a first terminal and a second terminal. The first terminal of each one of the second plurality of the fixed resistors are connected to a respective one of the row lines and the second terminals of the second plurality of the fixed resistors are connected together, thereby forming an additional column of the matrix.
In accordance with a further added feature of the invention, the printed circuit board, the first plurality of the fixed resistors, and the second plurality of the fixed resistors are not integral parts of the sensor mat.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of determining a resistance value of a force-dependent resistor of a sensor mat that includes steps of: providing a sensor mat having row lines, column lines, and a plurality of force-dependent resistors connected between the row lines and the column lines to form a matrix having rows and columns; providing a printed circuit board having a plurality of fixed resistors; providing each one of the first plurality of the fixed resistors with a first terminal and a second terminal; forming an additional row of the matrix by connecting the first terminal of each one of the plurality of the fixed resistors to a respective one of the column lines and by connecting the second terminal of each one of the plurality of the fixed resistors together; using the plurality of the fixed resistors to determine leakage currents effecting the column lines; and using the leakage currents to determine actual resistance values of the plurality of force-dependent resistors.